SnoRNAs require cis-acting nucleolar localization elements for their mature form to remain in the nucleolus during interphase, to regroup as a nucleolus at the end of mitosis, and to migrate to the nucleolus after their biosynthesis in the nucleoplasm. E1, E2, and E3 snoRNAs do not belong to the main class of snoRNAs since they do not have sequences that are present in most snoRNAs. E1 and E2 snoRNAs are essential for 18S rRNA formation and the three RNAs interact directly in vivo with unique segments of pre-rRNA. Human E1, E2, and E3 RNAs specifically localize to the nucleolus after they are injected into the cytoplasm of frog oocytes. Based upon these observations, the P.I. proposes that each of these RNAs have novel cis-acting nucleolar localization domains that interact with novel trans-acting localization factors. To determine the molecular mechanisms of the nucleolar localization of these snoRNAs the P.I. will identify the nucleolar localization elements of these snoRNAs using snoRNA mutational analysis in whole cells. In addition, the nucleolar localization proteins needed by these snoRNAs will be identified using competition experiments in injected cells, mobility shift assays and RNA-protein crosslinking studies.